According as a size rule has been continuously reduced in semiconductor integrated circuit design, a space between adjacent interconnections has been narrowed. Consequently, delay due to parasitic capacitance between interconnections has been relatively increased and visible deterioration in high-speed performance resulting from the delay has been revealed. Specifically, in a semiconductor integrated circuit, its signal delay in propagation through interconnection depends on a CR time constant of the interconnection (C: interconnection capacitance; R: interconnection resistance). Thus, there is concern that when reduced line-width of interconnection cause increase in an interconnection resistance and in addition, decreased line-space between interconnections cause increase in a capacitance between the interconnections, which lead to significant increase in a CR time constant of interconnection, a rate for signal transfer in interconnections may become insufficient for an increasing switching speed in a transistor composing a circuit. Conventionally, an aluminum alloy has been generally used as an interconnection material in a semiconductor integrated circuit. However, a further integrated circuit for higher-speed operation requires an interconnection material with a reduced resistance for avoiding increase in an interconnection resistance due to reduction in a line-width of interconnection. Therefore, a copper-base interconnection has been recently used.
On the other hand, for avoiding increase in a capacitance between interconnections, there has been investigated applicability of insulating film materials with a lower dielectric constant than a silica (SiO2) based insulating film which has been widely used as an insulating film between interconnections. As material with a lower dielectric constant which can be used as an insulating film between interconnections in a semiconductor device, applications of fluorinated silica (SiOF) and porous silica and further of organic polymer films (organic insulating films) have been made.
For example, a fluorinated silica has been already used in some of products having come into the market. However, increase in a fluorine content aiming at further reducing a dielectric constant of a fluorinated silica film itself may lead to such a further problem that hydrogen fluoride produced as a reaction product with water or hydrogen will give rise to erosion of an interconnection metal, or removal of fluorine atoms will result in increase in a dielectric constant. In addition, further progress in semiconductor integrated circuit technique requires much more reduction in dielectric constant of an interconnection insulating film so that a dielectric constant of about 3.3 achieved with a fluorinated silica (SiOF) film has not been sufficient any longer. Thus, the use of an insulating material with a very low dielectric constant of 3 or less has been paid attention. In this respect, porous silica is one of promising materials as it can give a dielectric constant of 2 or less, but on account of its structure, water condensation in micro-pores thereof may cause increase in its dielectric constant or lowering in a threshold bias for leakage. As it possesses lower mechanical strength owing to porous structure, there are considerable occasions of such a problem that it cannot bear up against a physical stress suffered at the steps of chemical mechanical polishing (CMP) or wire bonding.
It is thus highly demanded to develop an organic polymer film exhibiting excellent heat resistance and moisture resistance that are required in applying it to an interlayer insulating film that insulates multi-layered interconnections on a semiconductor integrated circuit from each other. It is essential for good moisture resistance in an organic polymer film that organic monomers used as composing units for the organic polymer have no hydrophilic groups. Furthermore, it is suggested to be desirable that when the organic polymer film is formed by the polymerizing reaction of organic monomers for its backbone, the process of polymerizing reaction are conducted with no step going through condensation reaction by dehydration which produces water. The term an “organic monomer” as used herein refers to a starting material used as a unit component for giving a desired organic macromolecule (organic polymer) through polymerization.
Examples of a conventional method for layering a functional organic polymer film such as an interlayer insulating film include spin-coating method where starting organic monomers are spin-coated and then polymerized in the coat layer to give a polymer film. The Spin-coating method is one of methods widely used for forming an organic polymer film. In this process, as the organic monomers are dissolved in a solvent for spin coating, at a layer-forming step following the step of forming a coat layer, the solvent contained therein is removed by evaporation while the remaining organic monomers are heated up to make progress in polymerization of the monomers with each other. Finally, the polymerization gives, for example, a film having two- or three-dimensional network structure or polymer film being composed of the organic monomers as component units. The organic polymer thus obtained is an insulating material and thus functions as an organic insulating film. The composition and the structure of the organic insulating film formed by the spin coating process depend on the structures of the organic monomers dissolved in the organic solvent used in spin coating and a content ratio of a plurality of organic monomers therein. Accordingly, in principle, it is impossible to make changes in a composition of an organic insulating film in its thickness direction.
In “REAL-TIME FT-IR STUDIES OF THE REACTION KINETICS FOR THE POLYMERIZATION OF DIVINYL SILOXANE BIS BENZOCYCLOBUTENE MONOMERS” (Material Research Symposium Proceedings Vol. 227, p. 103, 1991) T. M. Stokich Jr., W. M. Lee, R. A. Peters (hereinafter referred to as Reference 1), for example, there is a description about a process for forming a film made of an organic polymer having a three-dimensional molecular chain structure that is composed of divinylsiloxane bis(benzocyclobutene) monomer as frame units, as represented by chemical formula (IV) below; the process comprises the steps of spin-coating a solution of divinylsiloxane bis(benzocyclobutene) monomer in mesitylene as a solvent, pre-baking the coat layer at 100° C. to remove the solvent, mesitylene, thereof, and then heating up to 300° C. to 350° C. to initiate thermal ring-opening polymerization of the four-membered carbocyclic ring in the benzocyclobutene skeleton of the starting monomer molecule; Formula (IV): polymer film being composed of a divinylsiloxane bis(benzocyclobutene) as a frame unit

In the spin coating method, organic monomers are dissolved in an organic solvent, and then the resulted solution is spin-coated so that about 90% of the solution used at the spin coating step is flown out of a substrate. Thus, with respect to organic monomers used as its starting material, it is a method having a poor utilization efficiency thereof. As a result, the organic monomers used as its starting material have a relatively higher proportion in a production cost.
During the spin-coated film is heated in a baking furnace to evaporate the organic solvent and the coat is further heated at a higher temperature to form a desired organic polymer film by initiating polymerization of the organic monomers therein, if oxygen is present in the baking furnace, the oxygen molecule may react with a part of the organic monomers, which gives rise to an occasional failure in production of the desired organic polymer film. For preventing such a side reaction, the atmosphere of the whole system must be replaced with an inert gas such as nitrogen for eliminating oxygen molecules remaining in the baking furnace in advance. This treatment is a factor leading to an additional cost for production, which thus may be one of hurdles making cost reduction difficult.
Furthermore, it is predicted that oxygen dissolved in an organic solvent used may optionally react organic monomers during baking. For eliminating such possibility, the atmosphere during process comprising the steps of preparation of the solution and spin coating needs be strictly regulated. When using the spin coating method, it is difficult to strictly control the atmosphere throughout all the whole process. Furthermore, an organic solvent is used in spin coating of organic monomers as well as in coating of resist. Thus, although the process is carried out in a spin-coating chamber with local exhaust under a clean atmosphere, floating fine dust particles or fine particles of organic monomers dried to solid after scattering may happen to contaminate a spin-coated film formed. For a resist, such contaminants can be finally removed at the end of a series of steps, and therefore, if contaminating of fine particles occurs, they do not remain in a semiconductor device obtained. On the other hand, in the case of an organic polymer film in use for such an interlayer insulating film, the organic polymer formed has an abnormal microscopic structure around the fine particles, which may be an occasion for deterioration in quality of the organic insulating film, for example, local leak channels may be generated thereby after long-term operation, which will affect its high performance in protecting against leakage. Of course, in the case of spin coating, as large amount of a volatile solvent is used therein, the handling is carried out under a circumstance with local exhaust, which strives to draw back the organic solvent vaporized, but there is not few occasions that a very small amount thereof may get off, and thus spin coating method has an inherent problem such that it may load environment with a significant burden.
We have proposed a process by means of organic-monomer evaporation as for a process for forming a functional film of a organic polymer by utilizing a vapor growth method in Japanese Patent Application Laid-open No. JP 11-017006A. According to this method for vapor growth of an organic polymer film, it is a process comprising the steps of evaporating an organic monomer as a starting material, feeding the monomer molecule in vapor phase, and then thermally polymerizing the monomer molecules with each other on a substrate to obtain a film of an organic polymer thereby. FIG. 6 schematically shows an apparatus for film formation according to such a method for vapor growth of an organic polymer film using the vapor of an organic monomer directly vaporized as a starting material. An organic monomer 1 in a reservoir 55 is heated under a reduced pressure to vaporize. On the other hand, while a reaction chamber 51 is held under a reduced pressure by evacuating with a vacuum pump 50, the vaporized molecule of the organic monomer is fed into the reaction chamber 51 via a gasified material line 56. The molecules of organic monomer supplied adhere to the surface of a semiconductor substrate 53 where a semiconductor integrated circuit is formed in advance. At that time, the semiconductor substrate 53 is heated with a substrate heater 54 so that progress in polymerization reaction between the organic monomer molecules is made by a thermal energy at the temperature, and a cross-linked structure is built up therewith to form an organic insulating film 52. In the process for layered growth of an organic polymer film described deposition in Japanese Patent Application Laid-open No. JP 11-017006A, which is referred herein as to an organic monomer evaporation method, in contrast with the spin-coating method, no organic solvent is used in the process, and further as the layer growth is conducted in a reaction chamber with a reduced pressure, oxygen is not present in the atmosphere therein. Therefore, the method has an advantage that in principle it eliminates factors adversely affecting a quality of film, such as reaction with oxygen molecules and bubbles or voids associated with vaporization of an organic solvent remaining in a coat film, which are sometimes observed in the spin-coating method. However, there remains a technical problem that, when a substrate temperature is elevated for increasing a polymerization degree or increasing a polymerization rate, it heightens rate of desorption of organic monomer molecules once adsorbed, resulting in lowering an effective rate of adsorption onto a substrate, and as a result, aimed improvement in a growth rate cannot be achieved.